This application claims priority to Korean Patent Application No. 2005-79949, filed on Aug. 30, 2005 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates generally to complementary metal oxide semiconductor (CMOS) image sensors, and more particularly, to a CMOS image sensor having a reduced number of optical black pixel rows for a smaller size of the optical black area.
2. Background of the Invention
A CMOS image sensor is a photo-electric conversion device that converts an image into electrical signals. The conventional CMOS image sensor includes an active pixel array and an optical black area for converting an image projected onto the active pixel array into digital signals in response to a ramp signal.
When the CMOS image sensor outputs image signals, thermally generated electrons as well as electrons photo-electrically converted from the image are output from the active pixel array. For determining the quantity of just the photo-electrically converted electrons, the quantity of thermally generated electrons is subtracted from the measured total charge quantity.
The optical black area of the CMOS image sensor generates signals for indicating the quantity of the thermally generated electrons. The optical black area is covered with a metal layer to block out light from such an area. Thus, an optical black signal output from the optical black area indicates the quantity of only thermally generated electrons, and the optical black signal is also referred to as a dark signal.
An image signal output from the active pixel array indicates the quantity of both the photo-electrically converted electrons and the thermally generated electrons. Thus, the optical black signal is subtracted from such an image signal to generate a corrected image signal that reflects just the quantity of the photo-electrically converted electrons from the image.
In addition, the image signal from the active pixel array and the optical black signal from the optical black area are converted into digital signals using a ramp signal in a CDS (correlated double sampling) method. However, such a ramp signal also includes a noise component that commonly affects the digital image signal and the digital optical black signal. The noise component of the ramp signal may be removed when the digital optical black signal is subtracted from the digital image signal.
The conventional CMOS image sensor has an equal number of optical black pixel rows as the number of active pixel rows. Thus, a row of active pixels with a corresponding row of optical black pixels is sequentially activated with row select signals for generating corresponding image signals and optical black signals.
A separate respective optical black pixel row formed for each of the active pixel rows in the conventional CMOS image sensor results in a relatively large optical black area. Such a large optical black area disadvantageously reduces availability of area for the active pixel rows. Thus, a CMOS image sensor with reduced size of the optical black area is desired.